Alimbic Cannon

From Wikitroid, the Metroid wiki

When Gorea had all but extiguished the Alimbics, they found a way to lock him up forever. Constructing the Oubliette, they locked him in it. Then, realizing that he could escape, they used the weapon known as the Alimbic Cannon to open a portal to the Infinity Void. The cannon is operated by replacing all of the Octoliths (obtained from the Slenches and Cretaphids) in the Firing room and then it will fire into the center of the Alimbic's System and open up the door to the Oubliette where the final battle against Gorea commences.